Crystal and molecular structures of two silver(I) amidinates, including an unexpected co-crystal with a lithium amidinate

In the dimeric silver(I) amidinates [Ag{RC(NR′)2}]2 (R = Ph, cyclopropylalkynyl; R′ = Cy, iPr), a centrosymmetric planar Ag2N4C2 ring with strictly linearly coordinated silver ions is present. [Ag{cyclo-C3H5—C≡C–C(NCy)2}]2 forms a 1:1 co-crystal with the related lithium derivative [Li{cyclo-C3H5—C≡C–C(NCy)2}(THF)]2, in which the lithium component exhibits a typical dimeric ladder structure.


Chemical context
Anionic N-chelating donor ligands such as amidinates, [RC(NR) 2 ] À , and guanidinates, [R 2 NC(NR) 2 ] À , have gained tremendous importance in various fields of organometallic and coordination chemistry over the past two decades. Formally, amidinate anions are the nitrogen analogues of the carboxylate anions, while guanidinates are similarly related to the carbamates. However, in contrast to the carboxylates and carbamates, the steric properties of amidinates and guanidinates can be widely tuned through the use of different substituents, both at the outer nitrogen atoms as well as at the central carbon atom of the NCN unit. Both types of N-chelating ligands are often regarded as 'steric cyclopentadienyl equivalents' (Bailey & Pace, 2001;Collins, 2011;Edelmann, 2008Edelmann, , 2013. Meanwhile, amidinato and guanidinato complexes are known for virtually every metallic element in the Periodic Table ranging from lithium to uranium (Edelmann, 2008(Edelmann, , 2009(Edelmann, , 2012(Edelmann, , 2013Trifonov, 2010). Alkyl-substituted amidinate and guanidinate complexes of various metals have also been established as ALD and MOCVD precursors for the deposition of thin layers of metals, metal oxides, metal nitrides etc. (Devi, 2013). The most important starting materials in this field are lithium amidinates and guanidinates. Lithium amidinates are normally prepared in a straightforward manner by addition of lithium alkyls to N,N 0 -diorganocarbodiimides in a 1:1 molar ratio, while lithium guanidinates are formed when lithium-N,N-dialkylamides are added to N,N 0 -diorganocarbodiimides (Stalke et al., 1992;Aharonovich et al., 2008;Chlupatý et al., 2011;Nevoralová et al., 2013;Hong et al., 2013). On the other hand, silver(I) amidinates and guanidinates (Archibald et al., 2000;Lim et al., 2003;Whitehorne et al., 2011;Lane et al., 2014) are of significant importance as potential precursors for vapor deposition processes (Lim et al., 2003;Whitehorne et al., 2011), as precursors for silver nanoparticles (Cure et al., 2015), or as intermediates in silver-catalyzed amidination and guanylation reactions (Pereshivko et al., 2011;Okano et al., 2012;Li et al., 2015).

Structural commentary
Silver(I) compound 1 (Fig. 1) and silver moiety 2a (Fig. 2): Both silver(I) complexes exist as centrosymmetric dimers in the crystalline state. Compound 1 crystallizes without any solvent, and the molecular structure of moiety 2a was determined from the co-crystal 2 (2a Â 2b Â toluene). In both 1 and 2a, each of the two N atoms of the amidinate ligand coordinates to one Ag atom (coordination mode -N:N 0 ), and the Ag atoms adopt an almost linear coordination [1: N-Ag-N 170.58 (7) ; 2a: N-Ag-N 170.66 (5) ] by two N atoms of two symmetry-related amidinate ligands, leading to centrosymmetric dimers in each case. The Ag-N separations are very similar in both structures [1: 2.0959 (16) and 2.0965 (16) Å , 2a: 2.0908 (15) and 2.0916 (14) Å ]. An sp 2 hybridization can be assigned to the N atoms since the coordination environment is almost trigonal-planar. The C-N separations within the amidinate NCN fragment are virtually equal [1: twice 1.322 (3) Å , 2a: 1.329 (2) and 1.331 (2) Å ], indicating a typical delocalization of the negative charge. Through the mentioned connectivity pattern, a strictly planar C 2 N 4 Ag 2 eightmembered ring with a short AgÁ Á ÁAg contact is built [1: 2.6604 (3) Å , 2a: 2.6838 (3) Å ]. This constitution might be supported by some attractive d 10 -d 10 interaction between the Ag atoms that have been frequently discussed in the literature (for a review, e.g. see: Jansen, 1987). The molecular structures of the here discussed compounds are closely related to those of the most previously described copper(I) and silver(I) amidinates, namely [Cu 2 {RC(NR 0 ) 2 } 2 ] (R, R 0 = Me, n Bu; Li et al., 2005) and [M 2 {MeC(N i Pr) 2 } 2 ] (M = Cu, Ag). However, in the case of Ag{MeC(N i Pr) 2 }, also a trimeric structure [Ag 3 {MeC(N i Pr) 2 } 3 ] was observed (Lim et al., 2003). The bond lengths and angles involving the Ag atoms, viz. Ag-N and Ag-Ag distances and N-Ag-N angles, in the compounds discussed herein resemble those observed in the previously reported dimeric silver(I) amidinates. A dimerization or oligomerization under formation of linear N-M-N units is also typical for a broad ensemble of copper(I) and silver(I) complexes with other anionic nitrogen ligands, e.g.

Figure 1
The molecular structure of compound 1. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. [Symmetry code: ( 0 ) 2 À x, 2 À y, 1 À z.]

Figure 2
The molecular structure of moiety 2a, determined from the co-crystal 2. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. [Symmetry code: ( 0 ) 2 À x, 2 À y, 1 À z.] 2016). The silane diamide complexes [M 4 (ThioSila) 2 ] comprise a planar Si 2 N 4 M 2 ring that is structurally closely related to the C 2 N 4 M 2 ring in the dimeric amidinate complexes.
Lithium moiety 2b (Fig. 3): The molecular structure of 2b was determined from the above-mentioned co-crystal 2 (2a Â 2b Â toluene). Like the silver components 1 and 2a, the lithium moiety exists as a centrosymmetric dimer in the crystalline state. However, the molecular structure of 2b is considerably different, featuring a centrosymmetric Li 2 N 2 four-membered ring formed by -bridging coordination of one of the N atoms (N3). The Li-N distances within this ring are 2.033 (4)-2.261 (4) Å and therefore in the expected range. The coordination number of the mentioned N atom N3 is consequently raised to four and an sp 3 hybridization fits best to describe the bonding situation. The second N atom of the amidinate ligand (N4) is attached to only one Li atom with a shorter Li-N bond of 2.001 (4) Å , and its coordination environment is trigonal-planar like in the related silver components. Through this -N:N:N 0 -coordination mode of the amidinate ligands, a 'ladder' consisting of three fourmembered rings is formed. By coordination of a solvent THF molecule, a typical distorted tetrahedral coordination of the Li atom is completed. Just like in the case of the silver components 1 and 2a, the C-N bond lengths within the amidinate moiety are very similar with 1.321 (2) and 1.335 (2) Å . The structural motif of ladder-type dimers is typical for this class of compounds and has frequently been observed for most of the previously characterized lithium amidinates and guanidinates (Stalke et al., 1992;Snaith & Wright, 1995;Downard & Chivers, 2001, Brown et al., 2008.

Supramolecular features
In both of the presented crystal structures, there are no specific intermolecular interactions. In compound 1 (Fig. 4), the closest intermolecular contacts exist between phenyl groups and isopropyl groups [min. HCÁ Á ÁCH 3 3.79 (1) Å ]. In the co-crystal 2 (Fig. 5), four silver amidinate molecules (2a) are situated on the centres of the four unit-cell edges perpendicular to (001) and four lithium amidinate molecules (2b) on the four edges perpendicular to (010). The four remaining unit-cell edges perpendicular to (100) are occupied by four disordered toluene molecules. The closest intermolecular contacts exist between the cyclopropyl moieties of the silver complex and the toluene methyl groups [C6Á Á ÁC44 3.48 (1) Å ], followed by cyclopropyl-cyclopropyl contacts between silver amidinate and lithium amidinate molecules [C5Á Á ÁC24 3.57 (1)   The molecular structure of moiety 2a, determined from the co-crystal 2. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. [Symmetry code: ( 00 ) 2 À x, 1 À y, Àz.]

Figure 4
Crystal packing of dimeric silver(I) amidinate molecules in compound 1, viewed in a projection on (100).

Synthesis and crystallization
[Ag 2 {PhC(N i Pr) 2 } 2 ] (1) was obtained following a published procedure (Lim et al., 2003). Therefore, an in situ prepared solution of the lithium derivative Li{PhC(N i Pr) 2 } (Sroor et al., 2013) in THF was treated with a stoichiometric amount of silver(I) chloride at room temperature ( Fig. 6). Afterwards the solvent was removed in vacuo, the residue was extracted with toluene and the insoluble matter filtered off. After addition of an excess of n-pentane to the filtrate, large colorless crystals formed within few days at room temperature. 1  Single crystals of the co-crystal (2) with composition [Ag{c-C 3 H 5 -C C-C(NCy) 2 }] 2 (2a) Â [Li{c-C 3 H 5 -C C-C(NCy) 2 }(THF)] 2 (2b) Â toluene were serendipitously obtained in an attempt to prepare the pure silver(I) derivative 2a. The reaction of the in situ prepared lithium compound 2b (Sroor et al., 2013) with silver(I) chloride in THF analogous to the procedure described for compound 1 afforded a small quantity of colorless co-crystals of (2). Mp. = 393 K. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were fixed geometrically and refined using a riding model with U iso (H) = 1.2U eq (C). C-H distances in CH 3 groups were constrained to 0.98 Å , those in CH 2 groups to 0.99 Å and those in CH groups to 1.00 Å . All CH 3 groups were refined as freely rotating around the C-C vector.  (Sheldrick, 2015b), DIAMOND (Brandenburg, 1999) and publCIF (Westrip, 2010).

Figure 6
Synthesis of silver(I) amidinates from the related lithium derivatives.
For compound 2, the reflection (100) was partly obstructed by the beam stop and was therefore omitted from the refinement. The U ij components of the C atoms of the THF molecule (C41-C44) were restrained to be similar for atoms closer than 1.7 Å (SIMU restraint in SHELXL; the s.u. applied was 0.01 Å 2 ). The toluene molecule (C41-C44) is located on an inversion center. Consequently, the methyl group (C44) and the para-H atom (H64) are disordered over two positions and were refined with a constrained site occupancy factor of 0.5. The ipso-C and para-C atom (C42A and C42B) were refined to be equal (EXYZ and EADP restraints in SHELXL).

sup-1
Acta Cryst.  (Brandenburg, 1999); software used to prepare material for publication: publCIF (Westrip, 2010). Extinction correction: SHELXL2016 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00133 (9) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.